The instant invention relates to test fixtures which are utilized in the computerized testing of printed circuit board assemblies, and it more particularly relates to a bi-level test fixture which utilizes a motor driven shuttle plate actuator for movement of the shuttle plate between first and second testing positions.
Computerized test systems have generally been found to be highly effective for use in testing printed circuit board assemblies, particularly when testing large quantities of identical printed circuit board assemblies such as those which have been manufactured in mass production operations. In particular, computerized test systems have been found to be effective for both functional testing, wherein circuit boards are tested in a mode that simulates actual use, and incircuit testing wherein the components of circuit boards are individually tested.
Heretofore, specialized test fixtures have been available for performing both functional testing and incircuit testing. These specialized test fixtures are commonly referred to as bi-level test fixtures and they typically include two sets of spring-loaded contact probes of two different lengths which are installed so that they extend upwardly to two different heights above the probe plate, a diaphragm plate for supporting a circuit board above the probe plate, and a vacuum system for drawing the circuit board downwardly into contact with the probes on the probe plate. In a system of this type the higher set of probes is normally utilized for the functional testing circuit boards, and the lower set of probes is normally utilized for the incircuit testing. When a system of this type is operated in a functional test mode the diaphragm plate thereof is drawn downwardly to an upper test level in which the circuit board makes contact with only the higher probes, and when a system of this type is operated in an incircuit test mode the diaphragm plate is drawn further downwardly to a lower test level where the circuit board makes contact with both sets of probes.
The heretofore available bi-level test fixtures have utilized various known mechanisms for positioning the diaphragm plates thereof at the upper and lower testing levels thereof. One such mechanism comprises a thin sliding plate, commonly referred to as a "shuttle plate", which has a plurality of spaced projections or stop discs mounted on the upper surface thereof. In this regard, a shuttle plate of this type is normally disposed between the probe plate and the diaphragm plate of a fixture and it is adapted to be shifted back-and-forth between two test positions, one wherein the stop discs are aligned with corresponding holes in the bottom of the diaphragm or plate, and one wherein the stop discs are in nonaligned relation with the holes. When the stop discs are in nonaligned relation with the holes and the diaphragm plate is drawn downwardly, the stop discs engage the diaphragm plate to retain it in the upper testing position. When the stop discs are in aligned relation with the holes and the diaphragm plate is drawn downwardly, the stop discs pass into the corresponding holes to allow the diaphragm plate to be drawn downwardly to the lower testing position.
Most of the heretofore available bi-level test fixtures had either utilized solenoids or air cylinders for shifting the shuttle plates thereof back-and-forth. However, it has also been found that both of these apparatus have significant disadvantages which indicate a need for a more effective shifting apparatus. Specifically with regard to air cylinder actuators, it has been found that most computerized testing facilities do not have readily available compressed air supplies, and that in the facilities which do have readily available air supplies it is often very inconvenient to connect the air supplies to test fixtures. With regard to solenoid actuators, it has been found that because solenoids do not generate significant amounts of force for their size, relatively large solenoids are generally required to slide the shuttle plates of fixtures. It has been found that because of this test fixtures which include solenoid actuators often require the use of oversized fixture housings. Even further, it has been found that large solenoids require significant amounts of electrical power which normally cannot be accommodated with normal system relays. Still further, it has been found that when the power to the solenoids is turned off, the shuttle plates attached thereto are often freely slidable within fixtures, and that as a result, they can cause inadvertent damage thereto.